1. Field of the Invention
The invention relates to a device for polarization-specific examination of an optical system, having a detector part that comprises polarization detector means for recording the exit state of polarization of radiation emerging from the optical system, to an optical imaging system having such a device, and to an associated calibration method.
In particular, such a device can be used to determine how an optical system influences the state of polarization of optical radiation. These types of device are known for this purpose. The term optical system is to be understood in this case as any arrangement of one or more optical components which transmit and/or reflect the incident optical radiation, in particular including lenses and objectives constructed therewith. The term optical radiation is to be understood here as any desired electromagnetic radiation which is applied to the optical system under examination, for example visible light or UV radiation. Particularly widely used are ellipsometry methods and ellipsometry apparatuses in diverse forms. In order to describe the state of polarization and how it is influenced or changed by the optical system, use is made of suitable variables such as the Stokes parameters, the Müller matrix, the polarization matrix and the Jones matrix. Reference may be made to the relevant literature for details in this regard.
2. Description of the Related Art
As is known, it is possible for the purpose of determining the image quality of optics which image with high precision to make use of wavefront sensors with the aid of which deviations of the image-side wavefronts from the ideal imaging behavior can be determined very accurately. So-called shearing interferometers, for example, are in use for this purpose. A wavefront detection device based thereon is disclosed in laid-open specification DE 101 09 929 A1. This device is also suitable, in particular, for determining the image quality of projection objectives of microlithographic projection exposure machines, and includes means for providing a wavefront source, for example with a light guide and a shadow mask arranged at the output thereof, in the object plane of the optical imaging system under examination and a diffraction grating in the image plane conjugate to the object plane. Connected downstream of the diffraction grating is a spatially resolving radiation detector, for example in the form of a CCD chip, an interposed optical system imaging the interferogram produced by the diffraction grating onto the sensor surface of the detector. This type of wavefront sensor technology can examine the imaging system with the aid of the same radiation which is used by the imaging system in its normal operation. This type of wavefront sensor is therefore also denoted as an operational interferometer (OI) and can, in particular, be integrated in a module with the imaging system.
In German patent application 102 17 242.0, which is not a prior publication, a measuring device is described which can, in particular, be such an OI device and serves the purpose of interferometric measurement of an optical imaging system which is used for imaging a useful pattern, provided on a mask, into the image plane, the mask being arranged in the object plane for this purpose. It is proposed to implement the wavefront source for the interferometric measurement by means of a measuring pattern formed on the mask in addition to the useful pattern.
A further method, used in practice, of wavefront detection by high-precision imaging systems is represented by point diffraction interferometry, the basic principles of which are described in the relevant specialist literature—see, for example, D. Malacara, “Optical Shop Testing”, Chapter 3.7, John Wiley, New York, 1991. Specific discussions are provided in patent specifications U.S. Pat. No. 6,344,898 B1 and U.S. Pat. No. 6,312,373, and in the laid-open specifications JP 11-142291 and WO 02/42728. Further methods used for determining and/or correcting aberrations of high-precision imaging systems are, for example, the Shack-Hartmann method and the so-called Litel method, used by Litel Instruments—for the latter see, for example, patents U.S. Pat. No. 5,392,119, U.S. Pat. No. 5,640,233, U.S. Pat. No. 5,929,991 and U.S. Pat. No. 5,978,085. These interferometric and non-interferometric measuring methods can be used, in particular, to determine aberrations for projection objectives of microlithographic projection exposure machines.
In the case of modern high-precision imaging systems of high numerical aperture, used as microlithographic projection objectives, for example, the influence of the imaging system on the state of polarization of the radiation used can scarcely be neglected any longer. Thus, for example, polarization-induced effects on the image quality are produced by birefringence in the case of lenses made from calcium fluoride such as are frequently used for short wavelengths, and by polarization effects at deflecting mirrors. There is therefore a need to be able to determine the influencing of the state of polarization of optical imaging systems of high aperture as well as possible in quantitative terms, in order to draw conclusions on the polarization-dependent image quality.
Various devices for determining the influencing of the state of polarization by an optical system, and polarization analyzer arrangements suitable therefor are described in the older German patent application 103 04 822.7, which is not a prior publication and whose content is hereby incorporated in full by reference. The device described there and the method described there can be used for example, to determine the phase-reduced or complete Jones matrix of an examined optical imaging system, in particular also of high-aperture microlithographic projection objectives, in a pupil-resolved fashion. The polarization detector means of the detector part, which is arranged on the image side of the examined optical imaging system, there have a compensator polarizer unit, typically in the form of a λ/4 plate, with a downstream polarization analyzer element, typically in the form of a polarization beam splitter element, and an upstream, collimating optical system, in order to reshape the aperture beam emerging from the examined optical imaging system into a parallel beam required for the polarization analyzer element. Such a collimating optical system records a corresponding overall height of the detector part. Alternatively specified polarization detector means include small refractive or diffractive lenses with a downstream miniaturized polarization splitter cube. Such miniaturized optical components are however, relatively complicated to produce.
In the German article by J. S. Van Delden entitled Ortho-Babinet polarization-interrogating filter: an interferometric approach to polarization measurement, Optics Letters, Volume 28, No. 14, 15 Jul. 2003, page 1173, a description is given of an imaging Stokes polarimetry technique using an OBPI filter, with the aid of which all four Stokes parameters, and thus the state of polarization of the radiation can be determined over the entire image in a spatially resolved fashion without movable parts for imaging radiation incident in a quasi-parallel fashion. This OBPI method only requires a single picture record, and this is also denoted as snapshot polarimetry. The OBPI filter includes two retarder elements that in each case comprise a pair of uniaxial, linearly birefringent wedge elements, and are connected in series with the aid of wedge angles rotated by 90° to one another, and also a downstream linear polarizer. The two birefringent wedge plate retarders produce a change in the retardation of penetrating radiation that rises linearly in space in two dimensions. The downstream linear polarizer converts this retardation variation into a spatially oscillating brightness distribution. The result for prescribable, pure state of polarization of the incident radiation is different periodic structural patterns of the intensity distribution that serve as basic or reference patterns. It is then possible with the aid of the latter to determine the unknown state of polarization of an image of low-aperture imaging by reconstructing the fractions of the various reference patterns from the measured structural pattern by means of a suitable evaluation method. When the reference patterns are determined experimentally, they simultaneously also include some instrumentally induced, non-ideal influences, the result being to provide internal calibration.